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Post-quantum: .NET 11 RC2 adds composite ML-KEM, still experimental

Release candidate 2 of .NET 11, published October 13, introduces an experimental composite ML-KEM API. It combines a post-quantum algorithm and a classical one in a single key exchange.

By the automated monitoring of CyberiackUnder the editorial responsibility of Frédéric LENQUETTE

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Automated publication. This article was written by artificial intelligence, then checked by a second AI model that verifies the facts against primary sources. It was published without prior human review.

Translated from French by AI. The original French article is the reference version.

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Microsoft released the second release candidate (RC2) of .NET 11, its development platform, on October 13. The list of new features mentions experimental APIs for composite ML-KEM and HPKE. For .NET developers, it is another step toward hybrid post-quantum cryptography, which pairs an algorithm resistant to quantum computers with a classical algorithm.

What RC2 contains

According to the .NET libraries release notes, the new CompositeMLKem class combines a post-quantum key encapsulation mechanism, ML-KEM (standardized by NIST in FIPS 203), with a classical mechanism. Both contribute to the shared secret. If one of the two is ever broken, the other still protects the exchange. The CompositeMLKem.IsAlgorithmSupported method lets you check whether a given combination is available. The feature is only available on platforms that already support ML-KEM.

Three caveats apply:

  • Experimental status: to use the API, you must suppress the SYSLIB5006 diagnostic warning. Its interface could therefore still change.
  • HPKE not yet post-quantum: the HPKE (Hybrid Public Key Encryption) implementation in RC2 only supports classical Diffie-Hellman-based suites. ML-KEM suites are not yet supported. This API is also experimental (SYSLIB5009).
  • CMS: EnvelopedCms can now encrypt and decrypt with ML-KEM, but only with pure ML-KEM and only outside Windows.

An almost finalized IETF standard

Composite ML-KEM is based on an IETF draft. The IETF is the body that standardizes Internet protocols. This text, draft-ietf-lamps-pq-composite-kem, is at version 23, updated on October 14. It is in the final stretch of standardization but has not yet been published as an RFC. It defines twelve combinations that pair ML-KEM-768 or ML-KEM-1024 with RSA, NIST or Brainpool elliptic curves, or X25519 and X448. The hybrid approach matches the recommendations of ANSSI (France's national cybersecurity agency), which advises combining post-quantum and classical algorithms during the transition.

As a reminder, .NET 10 already offered the MLKem, MLDsa, SlhDsa and CompositeMLDsa (composite signatures) classes. According to Duende Software, only MLKem was considered stable there. These features require Linux with OpenSSL 3.5 or later, or a Windows version whose CNG cryptographic library supports post-quantum cryptography. macOS was not supported in .NET 10.

What to do

  • Test, but don't deploy: RC2 has a "go-live" license, but an experimental API has no place in long-lived production code.
  • Inventory the uses of RSA and ECDH in your .NET applications, to know where hybrid can fit in.
  • Check target platforms (OpenSSL version, Windows version) with the IsSupported methods before relying on these APIs.

Method: article written by artificial intelligence from public sources, facts verified by a second, independent AI model, published automatically without prior human review, under the editorial responsibility of Frédéric LENQUETTE. Spotted an error? Let us know.

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